The present invention relates to ceramic cutting tool materials and, in particular, to such cutting materials in which monocrystalline whiskers (hair crystals) based upon aluminum are homogeneously distributed in an oxide based ceramic matrix which leads to an improved strength and toughness without having a negative influence on the wear resistance to any appreciable extent.
Ceramic cutting tools have now been available for several decades but it has only been in the last ten years that they have achieved commercial importance for use in chip forming machining. The main reason for the limited use of ceramic cutting tools has been sudden and unexpected cutting insert failures because of inadequate strength and toughness.
During the last ten years the properties of ceramic cutting materials have been improved in many respects. As such, they have increased their relative market share in cutting of cast iron and (nickel-base) alloys with high heat resistance. In cutting of steel, which is by far the most dominating operation in the field of chip forming machining, the share of ceramic cutting inserts is still very modest. This is because a steel workpiece material places extreme and simultaneous demands on the strength, toughness and wear resistance of the cutting insert. To date, it has not been possible to fulfill these demands by known cutting tool materials.
Aluminum oxide based cutting tools are very sensitive to thermal cracking as aluminum oxide in itself has a relatively low thermal conductivity. This fact leads to a very short life in steel cutting, particularly under conditions having short engagement times and varying cutting depths.
To a certain extent the thermal properties have been improved by an addition of titanium carbide and/or titanium nitride which improves the thermal conductivity of the composite material. The addition of titanium carbide/nitride also increases the hardness of the material. In comparison with pure aluminum oxide materials, an increased life is therefore obtained in cutting of hard workpiece materials and in operations demanding resistance to thermal shocks. This type of material has, however, too poor a toughness behavior for a general use in cutting of steel.
A more recent development has been the addition of homogeneously dispersed fine-grained zirconium oxide particles in a matrix of aluminum oxide. Transformation of the "metastable" zirconium oxide particles during use increases both strength and toughness and will, thus, lead to a more predictable life.
The thermal properties of the foregoing type of material are, however, insignificantly better than those of pure aluminum oxide materials. This is why initiation and growth of thermally induced cracks are still great problems in practical cutting operations generating high cutting edge temperatures, such as in machining of steel.
It has recently been shown, e.g. U.S. Pat. No. 4,543,345 and EP-A-194 811, that alloying the SiC-whiskers, monocrystalline hair crystals, in a matrix of aluminum oxide leads to a greatly improved fracture toughness and strength. Ceramic cutting materials based upon this concept have exhibited very good performance in cutting of materials with high heat strength but in cutting of steel they have shown surprisingly short tool lives because of preferential attacks of the SiC-crystals. SiC has a high solubility in steel and is thus dissolved at a high rate leading to a weakening of the surface zone with accompanying high wear and risks of initiation of fractures.
EP-A-279 673 discloses aluminum oxide or aluminum nitride whiskers in a composite powder of alumina. DE 34 45 766 and DE 35 18 844 disclose alumina whiskers in a ceramic matrix. EP-A-283 454 discloses the use of whiskers of nitrides and/or carbides and/or borides of Ti and/or Zr in an oxide based matrix. The matrix may consist of oxides only (white ceramics) or it may additionally contain hard particles of carbides or nitrides (black ceramics). For compounds with extended homogeneity range, such as TiC, the disclosure is not restricted to only compounds with stoichiometric composition. EP-A-296 133 discloses the use of platelike "whiskers" (platelets) in a ceramic matrix and EP-A-295 228 discloses the use of a mixture of thin whiskers, thick whiskers and/or platelets in a ceramic matrix.
An object of the present invention is to obtain a ceramic material for chip forming machining of steel, in particular, where the above mentioned weaknesses of the known aluminum oxide based cutting materials have been eliminated. In materials according to the invention there is, thus, a unique combination of strength, toughness, thermal shock resistance and wear resistance, which has not been possible to obtain by previously known oxide based cutting materials.